Handovers (2G) and relocations (3G) in classic CS (Circuit Switched) mobile networks are ongoing calls that are moved between cells/service areas, BSC/RNCs (Base Station Controller/Radio Network Controller) and MSCs (Mobile Switching Center) if the concerned cells/service areas are served by two different BSC/RNCs and MSCs.
Handovers/relocations within classic CS mobile networks are always prioritized over normal calls (non-handover calls) in order to optimize the end user perception of the mobile service. This principle is based on the general opinion that it is more annoying for the end user to drop an ongoing call than having to redial when making a new call.
Prioritization of handovers/relocations over normal calls is performed by the target radio network (BSC/RNC) based on the type of request from the core network (MSC) or the UE (User Entity, also named Mobile Entity hereinafter). More specifically this means that the BSC/RNC always prioritizes radio resources for a Handover/Relocation Request message over a CM (Connection Management) Service Request message (a new MO (Mobile Originating) call), or a Paging Request message (a new MT (Mobile Terminating) call).
The prioritization of Handover/Relocation Request message over a CM Service Request message mechanism is needed in the BSC/RNC when there is high load or congestion in the target cell. The prioritization is also needed for minimizing the risk for dropped calls for fast moving UEs or generally for cases with sudden loss of coverage.
Handovers/relocations may also be new calls that are in the alerting state (ringing phase) or pre-alerting state (connecting phase) and considered as new calls from the end user point of view. However, for simplicity this application treats all handover/relocation call states (established, alerting and pre-alerting) equally.
Dual Radio Voice Call Continuity (DRVCC) is a concept for handover between different types of radio access networks, particularly between the non-3GPP WiFi access and the 3GPP based 2G and 3G radio accesses. A Dual Radio UE has both the 3GPP radio and the non-3GPP WiFi radio running active in parallel and can therefore easily transfer the call from the one access to the other. DRVCC does not require complex handover procedures in the network such as handling measurement reports, preparing target access and ordering the handover. The main complexity related to the DRVCC handover, including the decision to make a handover, is instead handled by the UE. When a DRVCC UE is about to lose WiFi coverage during a conversation it makes a new call in the 3GPP access by using standard call procedures for a mobile originating call. The conversation continues in the 3GPP access.
Enhanced Multi-Level Precedence and Pre-emption (eMLPP) is a concept for allocating priority to certain calls in the network based on either the subscription (e.g. gold/silver/bronze subscription etc.) or call type (e.g. emergency call). The priority is mainly aimed for the radio access network but it can also be used in the core network. If priority has been allocated to the originating subscriber of the call, this priority is also applied for the terminating subscriber.
The eMLPP priority information consists of a BSSMAP/RANAP priority level, a pre-emption vulnerability indicator, a pre-emption capability indicator and a queuing allowed indicator.
Ongoing WiFi calls will not be prioritized over new CS calls when moving into CS during DRVCC handover which means that operators' call drop call rate will be higher for WiFi calls than for classic CS calls. This is because the initial intention with the DRVCC handover mechanism in the standardization was to simplify the introduction of the handover by making it transparent to the receiving CS network. Instead of introducing complex handover procedures between the WiFi and CS network domain the DRVCC handover is reusing legacy call setup procedures when setting up the new handover call leg towards the CS network. This simplification unfortunately means that neither the MSC nor the 2G/3G RAN know if a new call request is for a new call or for a DRVCC handover of an already ongoing WiFi call.